Sorry these pictures are so bad, I don't know why I just couldn't get a good one.
There's ALOT of them, they just swarm over plants and over the deck stairs. I don't know if they're actually harming any plants b/c I didn't observe any bite marks but I think I did see them eating my miniature orange tree when I put it out last year.
It doesn't look like the carpenter bees here, as they have one yellow stripe compaired to two for the Bumbles.
It looks like one of the ground dwelling spider eating wasps.
What size is it, compared to a Bumble Bee?
Hee - thanks Heather. Night Bloom came over here to visit.
CaptMicha - these look like some kind of digger wasp to me - probably family Scoliidae or Sphecidae - but they do seem to be some kind of predatory wasp. Generally these catch and paralyze insects, usually caterpillars and/or crickets, and then bury them in a hole in the ground. Eggs are laid on the insect(s) and the developing wasp larva eats those.
When a whole bunch of them are out and about, they are usually emerging from the ground and looking for mates. If they were on your orange tree, they might have been searching for nectar as fuel for flight.
Some species of digger wasps are small, and males won't have stingers. Only the females need them to paralyze the prey. The males only have to find females and mate - no stinger needed.
This will also be the case with velvet ants (family Mutillidae) which are actually wasps. The females of those have stingers but don't have wings. It's opposite with the males. Usually velevt ants have either red or yellow coloration on them. Your's looks more like one of the families that I mentioned above, but I thought I'd give a picture of a velevt ant anyway, just in case it could be your guy.
Oh, I should've mentioned that the coloration in velvet ants does vary.
Also, I should have mentioned that smaller wasps might not bother your caterpillars - keep an eye out just in case - but smaller ones tend to go for spiders and smaller stuff. They also don't tend to go for fuzzy caterpillars or ones with "barbs" (like Fritillary caterpillars) I have lots of wasps in my gardens and they pay no attention to my fritillary cats at all.